DESCRIPTION: The principal investigator proposes to elucidate 1) the actions of the gas nitric oxide (NO) on neuronal processing in the primary visual cortex, and 2) the mechanisms that underlie those actions. Recent reports, some from the principal investigator s lab, suggest that NO might play an important role in short- and long term modulation of signal transfer characteristics of cortical circuits. The proposed experiments are a natural extension of the work in the previous grant period, in which in vitro experiments provided strong evidence for the role of NO in synaptic plasticity. In the next grant period the principal investigator wishes to extend the work to the in vivo preparation. Five specific hypotheses will be tested in an elaborate and careful research program, to demonstrate that NO in the living cat cortex modulates visual responses, that this modulation is down-stream from NMDA receptor activation, and that such modulation can have both transient and more long-lasting effects.